Semiconductor structures for latch-up suppression and methods of forming such semiconductor structures

ABSTRACT

Semiconductor structures and methods for suppressing latch-up in bulk CMOS devices. The semiconductor structure comprises a shaped-modified isolation region that is formed in a trench generally between two doped wells of the substrate in which the bulk CMOS devices are fabricated. The shaped-modified isolation region may comprise a widened dielectric-filled portion of the trench, which may optionally include a nearby damage region, or a narrowed dielectric-filled portion of the trench that partitions a damage region between the two doped wells. Latch-up may also be suppressed by providing a lattice-mismatched layer between the trench base and the dielectric filler in the trench.

FIELD OF THE INVENTION

The invention relates generally to semiconductor structures and methodsand, in particular, to methods for reducing or suppressing latch-up inbulk complementary metal-oxide-semiconductor device structures andsemiconductor structures fabricated by these methods.

BACKGROUND OF THE INVENTION

Complementary metal-oxide-semiconductor (CMOS) technologies integrate P-and N-channel field effect transistors (FETs) to form an integratedcircuit on a single semiconductor substrate. Latch-up, which isprecipitated by unwanted transistor action of parasitic bipolartransistors inherently present in bulk CMOS devices, may be asignificant issue for bulk CMOS technologies. The unwanted parasitictransistor action, which has various triggers, may cause failure of bulkCMOS devices. For space-based applications, latch-up may be induced bythe impingement of high energy ionizing radiation and particles (e.g.,cosmic rays, neutrons, protons, alpha particles). Because the integratedcircuit cannot be easily replaced in space flight systems, the chipfailure may prove catastrophic. Hence, designing bulk CMOS devices witha high tolerance to latch-up is an important consideration for circuitoperation in the natural space radiation environment, as well asmilitary systems and high reliability commercial applications.

Bulk CMOS device designs may be adjusted to increase latch-up immunity.For example, latch-up immunity may be increased in 0.25 micron devicetechnologies by building bulk CMOS devices on epitaxial substrates(e.g., a p-type epitaxial layer on a highly-doped p-type substratewafer). Highly-doped substrate wafers provide excellent current sinksfor currents that, if unabated, may initiate latch-up. However,epitaxial substrates are expensive to produce and may increase thedesign complexity of several critical circuits, such as electrostaticdischarge (ESD) protective devices.

Guard ring diffusions represent another conventional approach forsuppressing latch-up. However, guard ring diffusions are costly becausethey occupy a significant amount of active area silicon real estate. Inaddition, although guard ring diffusions collect a majority of theminority carriers in the substrate, a significant fraction may escapecollection underneath the guard ring diffusion.

Semiconductor-on-insulator (SOI) substrates are recognized by thesemiconductor community as generally free of latch-up. However, CMOSdevices are expensive to fabricate on an SOI substrate, as compared to abulk substrate. Furthermore, SOI substrates suffer from various otherradiation-induced failure mechanisms aside from latch-up. Anotherdisadvantage is that SOI devices do not generally come with a suite ofASIC books that would enable simple assembly of low-cost designs.

Conventional CMOS devices are susceptible to latch-up generally becauseof the close proximity of N-channel and P-channel devices. For example,a typical CMOS device fabricated on a p-type substrate includes aP-channel transistor fabricated in an N-well and an N-channel transistorfabricated in a P-well. The opposite conductivity N- and P-wells areseparated by only a short distance and adjoin across a well junction.This densely-packed bulk CMOS structure inherently forms a parasiticlateral bipolar (PNP) structure and parasitic vertical bipolar (NPN)structure. Latch-up may occur due to regenerative feedback between thesePNP and NPN structures.

With reference to FIG. 1, a portion of a standard triple-well bulk CMOSstructure 30 (i.e., CMOS inverter) includes a P-channel transistor 10formed in an N-well 12 of a substrate 11, an N-channel transistor 14formed in a P-well 16 of the substrate 11 that overlies a buried N-band18, and a shallow trench isolation (STI) region 20 separating the N-well12 from the P-well 16. Other STI regions 21 are distributed across thesubstrate 11. The N-channel transistor 14 includes n-type diffusionsrepresenting a source 24 and a drain 25. The P-channel transistor 10 hasp-type diffusions representing a source 27 and a drain 28. The N-well 12is electrically coupled by a contact 19 with the standard power supplyvoltage (V_(dd)) and the P-well 16 is electrically coupled by a contact17 to the substrate ground potential. The input of the CMOS structure 30is connected to a gate 13 of the P-channel transistor 10 and to a gate15 of the N-channel transistor 14. The output of CMOS structure 30 isconnected to the drain 28 of the P-channel transistor 10 and the drain25 of the N-channel transistor 14. The source 27 of the P-channeltransistor 10 is connected to V_(dd) and the source 24 of the N-channeltransistor 14 is coupled to ground. Guard ring diffusions 34, 36encircle the CMOS structure 30.

The n-type diffusions constituting the source 24 and drain 25 of theN-channel transistor 14, the isolated P-well 16, and the underlyingN-band 18 constitute the emitter, base, and collector, respectively, ofa vertical parasitic NPN structure 22. The p-type diffusionsconstituting the source 27 and drain 28 of the P-channel transistor 10,the N-well 12, and the isolated P-well 16 constitute the emitter, base,and collector, respectively, of a lateral parasitic PNP structure 26.Because the N-band 18 constituting the collector of the NPN structure 22and the N-well 12 constituting the base of the PNP structure 26 areshared and the P-well 16 constitutes the base of the NPN structure 22and also the collector of the PNP structure 26, the parasitic NPN andPNP structures 22, 26 are wired to result in a positive feedbackconfiguration.

A disturbance, such as impinging ionizing radiation, a voltage overshooton the source 27 of the P-channel transistor 10, or a voltage undershooton the source 24 of the N-channel transistor 14, may result in the onsetof regenerative action. This results in negative differential resistancebehavior and, eventually, latch-up of the bulk CMOS structure 30. Inlatch-up, an extremely low-impedance path is formed between emitters ofthe vertical parasitic NPN structure 22 and the lateral parasitic PNPstructure 26, as a result of the bipolar bases being flooded withcarriers. The low-impedance state may precipitate catastrophic failureof that portion of the integrated circuit. The latched state may only beexited by removal of, or drastic lowering of, the power supply voltagebelow the holding voltage. Unfortunately, irreversible damage to theintegrated circuit may occur almost instantaneously with the onset ofthe disturbance so that any reaction to exit the latched state isbelated.

What is needed, therefore, is a semiconductor structure and fabricationmethod for modifying standard bulk CMOS device designs that suppresseslatch-up, while being cost effective to integrate into the process flow,and that overcomes the disadvantages of conventional bulk CMOSsemiconductor structures and methods of manufacturing such bulk CMOSsemiconductor structures.

SUMMARY OF THE INVENTION

The present invention is generally directed to semiconductor structuresand methods that improve latch-up immunity or suppression in standardbulk CMOS device designs, while retaining cost effectiveness forintegration into the process flow forming the P-channel and N-channelfield effect transistors characteristic of bulk CMOS devices. Inaccordance with an embodiment of the present invention, a semiconductorstructure comprises a substrate of a semiconductor material and firstand second doped wells formed in the semiconductor material of thesubstrate. The second doped well is disposed adjacent to the first dopedwell. A dielectric-filled trench is defined in the substrate between thefirst and second doped wells. The trench has a base, first sidewallsintersecting a top surface of the substrate, and second sidewallsdisposed between the base and the first sidewalls. The second sidewallshave a wider separation than the first sidewalls.

In accordance with another embodiment of the present invention, asemiconductor structure comprises a substrate of a semiconductormaterial and first and second doped wells formed in the semiconductormaterial of the substrate. The second doped well is disposed adjacent tothe first doped well along a well junction. A dielectric-filled trenchis defined in the substrate between the first and second doped wells.The trench includes a base, first sidewalls intersecting a top surfaceof the substrate, and second sidewalls between the base and the firstsidewalls. The second sidewalls have a narrower separation than thefirst sidewalls. The semiconductor material of the substrate borderingthe second sidewalls includes a damage region comprisingnon-monocrystalline semiconductor material. The base of the trench is ata greater depth than the damage region for interrupting the continuityof the non-monocrystalline semiconductor material across the welljunction.

In accordance with another embodiment of the present invention, asemiconductor structure comprises a substrate of a first materialcharacterized by semiconducting properties, first and second doped wellsformed in the substrate, a trench defined in the substrate between thefirst and second doped wells, and a dielectric material filler in thetrench. The second doped well is disposed adjacent to the first dopedwell. The trench includes a base and sidewalls intersecting a topsurface of the substrate. A layer of a second material is disposedbetween the first material at the base of the trench and the dielectricmaterial filler. The first and second materials have a crystal latticeconstant difference sufficient to increase carrier recombinationvelocity.

In another embodiment of the present invention, a method is provided forfabricating a semiconductor structure in a substrate of semiconductormaterial. The method comprises forming a trench in the semiconductormaterial with a first sidewall and a second sidewall each disposedbetween a base of the trench and a top surface of the substrate. Themethod further comprises forming an oxygen-enriched region in thesemiconductor material of the substrate bounding the first sidewall ofthe trench near the base and converting the oxygen-enriched region to anoxide region.

In yet another embodiment of the present invention, a method is providedfor fabricating a semiconductor structure in a substrate ofsemiconductor material. The method comprises forming a trench in thesemiconductor material with first sidewalls extending from a base towarda top surface of the substrate and forming a damage region comprisingnon-monocrystalline semiconductor material at a first depth in thesubstrate below the base of the first trench. The method furthercomprises forming a second trench registered with the first trench andhaving second sidewalls between the base of the first trench to a seconddepth greater than the first depth. The second trench partitions thedamage region such that the non-monocrystalline semiconductor materialis discontinuous.

In yet another embodiment of the present invention, a method is providedfor fabricating a semiconductor structure in a substrate ofsemiconductor material. The method comprises forming a trench in thesemiconductor material with sidewalls extending from a base toward a topsurface of the substrate and forming an etch mask on the sidewalls. Themethod further comprises etching the trench to increase a depth of thebase from the top surface using an isotropic etchant that removes thesemiconductor material of the substrate bordering the trench below theetch mask to widen the sidewalls of the trench below the etch mask.

In another embodiment of the present invention, a method is provided forfabricating a semiconductor structure in a substrate of a first materialcharacterized by semiconducting properties. The method comprises forminga trench in the first material with sidewalls between a base and a topsurface of the substrate and forming a layer of a second material on thebase of the trench that has a crystal lattice constant difference incomparison with the first material sufficient to increase carrierrecombination velocity in the first material adjacent to the base.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the invention and,together with a general description of the invention given above and thedetailed description of the embodiments given below, serve to explainthe principles of the invention.

FIG. 1 is a diagrammatic view of a portion of a substrate with a bulkCMOS device constructed in accordance with the prior art.

FIGS. 2-5 are diagrammatic views of a portion of a substrate at variousfabrication stages of a processing method in accordance with anembodiment of the present invention

FIG. 5A is a top view of the substrate portion at the fabrication stageof FIG. 5.

FIG. 6 is a diagrammatic view of the portion of the substrate at afabrication stage subsequent to the fabrication stage of FIG. 5.

FIGS. 7-12 are diagrammatic views of a portion of a substrate at variousfabrication stages of a processing method in accordance with analternative embodiment of the present invention.

FIGS. 13-15 are diagrammatic views of a portion of a substrate atvarious fabrication stages of a processing method in accordance with analternative embodiment of the present invention.

FIG. 16 is a diagrammatic view similar to FIG. 14 depicting a portion ofa substrate at a fabrication stage of a processing method in accordancewith an alternative embodiment of the present invention.

FIG. 17 is a diagrammatic view similar to FIG. 13 depicting a portion ofa substrate at a fabrication stage of a processing method in accordancewith an alternative embodiment of the present invention.

FIG. 18 is a diagrammatic view similar to FIG. 3 depicting a portion ofa substrate at a fabrication stage of a processing method in accordancewith an alternative embodiment of the present invention.

DETAILED DESCRIPTION

The present invention provides an isolation region that limits theeffect of the vertical parasitic NPN structure and the lateral parasiticPNP structure responsible for latch-up in triple-well bulk CMOS devices.The invention is advantageously implemented in the context of bulk CMOSdevices where pairs of N-channel and P-channel field effect transistorsare formed adjacent to each other in a P-well within an N-band and anN-well, respectively, and the P-well is isolated from the N-well by ashallow trench isolation (STI) region. Specifically, the latchupimmunity of a standard bulk CMOS triple well structure is improved bymodifying the geometry of the STI region or by selectively adding damageregions to the STI region in a manner that significantly reduces thesusceptibility to latch-up. The present invention will now be describedin greater detail by referring to the drawings that accompany thepresent application.

With reference to FIG. 2, a bulk substrate 40 of a monocrystallinesemiconductor material is obtained. Substrate 40 may include alow-defect epitaxial layer for device fabrication that is grown by anepitaxial growth process, such as chemical vapor deposition (CVD) usinga silicon source gas (e.g., silane). Substrate 40 may be a singlecrystal silicon wafer containing a relatively light concentration of adopant providing p-type conductivity. For example, substrate 40 may belightly doped with 5×10¹⁵ cm⁻³ to 1×10¹⁷ cm⁻³ of a p-type dopant, suchas boron, by in situ doping during deposition of the epitaxial layer.

A pad structure 42 is formed on a top surface 41 of the substrate 40.The pad structure 42 includes a first pad layer 44 and a thinner secondpad layer 46 separating the first pad layer 44 from the substrate 40.The constituent material(s) of pad layers 44, 46 advantageously etchselectively to the semiconductor material constituting substrate 40. Thefirst pad layer 44 may be a conformal layer of nitride (Si₃N₄) formed bya thermal CVD process like low pressure chemical vapor deposition(LPCVD) or a plasma-assisted CVD process. The second pad layer 46 may besilicon oxide (SiO₂) grown by exposing substrate 40 to either a dryoxygen ambient or steam in a heated environment or deposited by athermal CVD process. The second pad layer 46 may operate as a bufferlayer to prevent any stresses in the material constituting the first padlayer 44 from causing dislocations in the semiconductor material ofsubstrate 40.

Shallow trenches 48 are formed in the semiconductor material ofsubstrate 40 by a conventional lithography and subtractive etchingprocess that utilizes a shallow trench pattern imparted in the padstructure 42 (FIG. 2) or, optionally, in a patterned hard mask (notshown) formed on pad structure 42. The shallow trench pattern may becreated in pad structure 42 by applying a photoresist (not shown) on padlayer 44, exposing the photoresist to a pattern of radiation to create alatent shallow trench pattern in the photoresist, and developing thelatent shallow trench pattern in the exposed photoresist. An anisotropicdry etching process, such as reactive-ion etching (RIE) or plasmaetching, may then be used to transfer the trench pattern from thepatterned resist into the pad layers 44, 46. The etching process, whichmay be conducted in a single etching step or multiple etching steps withdifferent etch chemistries, removes portions of the pad structure 42visible through the trench pattern in the patterned resist and stopsvertically on the substrate 40. After etching is concluded, residualresist is stripped from the pad structure 42 by, for example, plasmaashing or a chemical stripper.

The shallow trench pattern is then transferred from the patterned padlayer 44 into the underlying substrate 40 with an anisotropic dryetching process. The anisotropic dry etching process may be constitutedby, for example, RIE, ion beam etching, or plasma etching using an etchchemistry (e.g., a standard silicon RIE process) that removes theconstituent semiconductor material of substrate 40 selective to thematerials constituting the pad layers 44, 46. Each of the shallowtrenches 48 defined in the semiconductor material of substrate 40includes opposite sidewalls 50, 52, which are substantially mutuallyparallel and oriented substantially perpendicular to the top surface 41of substrate 40, that extend into the substrate 40 to a bottom surfaceor base 54.

Energetic ions, as indicated diagrammatically by singled-headed arrows56, are introduced by an ion implantation process into the substrate 40to create an oxygen-enriched or oxygen implanted region 58 proximate toand just beneath the base 54 of each shallow trench 48. The energeticions 56, which are generated from a source gas, are directed to impingethe top surface 41 of the substrate 40 at normal or near-normalincidence, although the invention is not so limited. The ions 56 may beimplanted with the substrate 40 at or near room or ambient temperature,although the present invention is not so limited.

The ions 56 lose energy via scattering events with atoms and electronsin the semiconductor material constituting substrate 40 as the ions 56penetrate the substrate 40. The ions 56 eventually dissipate all oftheir initial kinetic energy and stop in the substrate 40 to produce theoxygen implanted regions 58. The stopped ions 56 in the oxygen implantedregions 58 are characterized by a depth profile distributed about aprojected range, which is measured as a perpendicular distance of thedamage peak from the top surface 41. The depth profile is characterizedby a range straggle, which represents a deviation or second moment ofthe stopped ions 56 about the projected range. Essentially all of theimplanted ions 56 come to rest in the semiconductor material ofsubstrate 40 within a distance of three times the range straggle fromthe projected range. The implanted ions 56 also have a lateral stragglethat causes side edges 60, 62 of the oxygen implanted regions 58 toextend beyond the sidewalls 50, 52 of each shallow trench 48.

The ions 56 may originate from a source selected to provide, whenionized and accelerated to impart kinetic energy, oxygen ions. Theimplanted species may be either charged atomic oxygen ions (O⁺) ormolecular ions (O⁺²). Advantageously, the peak atomic concentration forthe implanted ions 56 in the oxygen implanted regions 58 may be in therange of 5×10¹⁹ cm⁻³ to 5×10²¹ cm⁻³ and, in certain embodiments, may beas low as 5×10¹⁸ cm⁻³ to provide the requisite oxygen concentration. Forexample, a suitable dose of implanted O⁺may range from 1×10¹⁴ cm⁻² to5×10¹⁶ cm⁻² at a kinetic energy between about 10 keV and about 50 keV,although the invention is not so limited. The present inventioncontemplates other implant conditions, i.e., energy and dose, may beused that are capable of forming oxygen implanted regions 58 insubstrate 40. The ions 56 are implanted across the top surface 41 of theentire substrate 40, although certain regions of substrate 40 may beoptionally protected by a block mask during implantation. Ions of anoxidation rate enhancing atomic species, such as germanium (Ge), silicon(Si), or arsenic (As) for n-well applications, or boron difluoride (BF₂)for p-well applications, may be co-implanted with ions 56. A block mask(not shown) of, for example, photoresist may protect a portion of thesubstrate 40 during the ion implantation process.

With reference to FIG. 3 in which like reference numerals refer to likefeatures in FIG. 2 and at a subsequent fabrication stage, spacers 64, 66are formed on the sidewalls 50, 52 of each shallow trench 48. Spacers64, 66 may be defined from a conformal layer (not shown) of a dielectricmaterial, such as 5 nm to 15 nm of nitride deposited by a CVD process,that is anisotropically etched using a reactive ion etch (RIE) or plasmaetching process. The etching process removes the material of theconformal layer (not shown) primarily from horizontal surfaces selectiveto (i.e., with a significantly greater etch rate than) the constituentsemiconductor material of substrate 40. The base 54 of each shallowtrench 48 is exposed after the spacers 64, 66 are formed.

With reference to FIG. 4 in which like reference numerals refer to likefeatures in FIG. 3 and at a subsequent fabrication stage, a majority ofeach of the oxygen implanted regions 58 (FIG. 3) is converted by athermal oxidation process to one of a plurality of oxide regions 68 eachconsisting of an oxide (e.g., silicon dioxide). The thermal oxidationprocess may be performed in a dry or wet oxidizing ambient atmosphereand at a temperature ranging from about 750° C. to about 1100° C. Theoxidizing species from the oxidizing ambient atmosphere penetrates thesubstrate 40 through the exposed base 54 of each shallow trench 48 inorder to reach the oxygen implanted regions 58. Spacers 64, 66 protectthe sidewalls 50, 52 of each shallow trench 48 against unwantedoxidation. The implanted oxygen in the oxygen implanted regions 58 (FIG.3) enhances the oxidation rate for the corresponding semiconductormaterial of substrate 40 when exposed to the oxidizing species from theoxidizing ambient atmosphere.

The perimeter of each oxide region 68 defines a curved boundary 69 thatextends laterally or horizontally of the sidewalls 50, 52 of eachshallow trench 48 because of the lateral oxide growth during the thermaloxidation process. The lateral extent of the concave boundary 69 of eachoxide region 68 roughly coincides with the side edges 60, 62 of theoxygen implanted regions 58 or may be slightly narrower than the sideedges 60, 62. Each oxide region 68 defines a degree of undercut relativeto the sidewalls 50, 52 and the degree of undercut increases withincreasing energy of the implanted ions 56. The lateral oxide growthdefines lateral extensions 73, 75 of STI regions 74 (FIG. 5),effectively defining sidewalls 70, 72 that widen each shallow trench 48relative to sidewalls 50, 52, and increases the depth of base 54relative to the top surface 41. The base 54 of each shallow trench 48 iseffectively deepened to a greater depth by the oxidation process formingoxide regions 68 as the remaining open volume in each shallow trench 48is filled with dielectric material in a subsequent fabrication stage.

A portion of each oxygen implanted region 58 (FIG. 3) may remain, afterthe thermal oxidation process, as a damage region 71 proximate to thecurved boundary 69 of the corresponding oxide region 68. The stopping ofthe ions 56 (FIG. 2) implanted in substrate 40 damages the constituentsemiconductor material to form non-monocrystalline semiconductormaterial confined within damage region 71. Energy transferred by nuclearcollisions between ions 56 and target atoms in the substrate 40displaces those target atoms from their original lattice sites and, as aconsequence, permanently damages the semiconductor material of thesubstrate 40. When each individual ion 56 displaces a target atom of thesubstrate 40 in a nuclear collision, a recoil cascade is initiated thatdissipates the transferred kinetic energy by collisions with othertarget atoms. The recoil cascade generates additional vacancies andinterstitial atoms in the lattice structure of substrate 40 dispersedamong the atoms in the crystalline lattice structure remaining onregular lattice sites. The damage in the damage region 71 may compriseextended crystal lattice defects that are larger than point defects anddisrupt long range order, or may render the crystalline structureamorphous. The crystalline damage in the damage region 71 coincidesapproximately with the depth profile of the stopped ions 56 and isstable in that the damage region 71 remains after subsequent fabricationsteps.

With reference to FIGS. 5 and 5A in which like reference numerals referto like features in FIG. 4 and at a subsequent fabrication stage, theshallow trenches 48 are filled with amounts of an insulating ordielectric material, such as a high-density plasma (HDP) oxide ortetraethylorthosilicate (TEOS), deposited across the pad layer 44 andplanarized by, for example, a CMP process. An optional high temperatureprocess step may be used to density the TEOS fill. The pad structure 42is removed by a planarization process to define shallow trench isolation(STI) regions 74 in the substrate 40 having a top surface substantiallyco-planar or flush with the top surface 41 of substrate 40.

With reference to FIG. 6 in which like reference numerals refer to likefeatures in FIG. 5 and at a subsequent fabrication stage, standard bulkCMOS processing follows, which includes formation of a triple-wellstructure consisting of an N-well 76, a P-well 78, and a deep buriedN-well or N-band 80 in the substrate 40. The buried N-band 80 supplieselectrical isolation for the P-well 78. This triple-well constructionpermits the optimization of bias potentials for both N- and P-wells 76,78. The P-well 78 is arranged between the N-band 80 and the top surface41.

The N-well 76, as well as other N-wells (not shown) dispersed across thesubstrate 40, are likewise formed by patterning a mask layer (not shown)applied on the top surface 41 with techniques known in the art, andimplanting an appropriate n-conductivity type impurity into thesubstrate 40 in unmasked regions. The N-band 80, as well as otherN-bands (not shown) dispersed across the substrate 40, are formed bypatterning another mask layer (not shown), such as a photoresist,applied on top surface 41 and implanting an appropriate n-conductivitytype impurity into the substrate 40 in this set of unmasked regions. TheP-well 78, as well as other P-wells (not shown) dispersed across thesubstrate 40, are likewise formed by patterning another mask layer (notshown) applied on top surface 41 and implanting an appropriatep-conductivity type impurity into the substrate 40 in this set ofunmasked regions. Typically, the P-well 78 is formed by counterdopingthe N-band 80 and has an opposite conductivity type from the N-well 76and N-band 80. Generally, the dopant concentration in the N-well 76ranges from about 5.0×10¹ cm⁻³ to about 7.0×10¹⁸ cm⁻³, the dopantconcentration in the P-well 78 ranges from about 5.0×10¹⁷ cm⁻³ to about7.0×10¹⁸ cm⁻³, and the dopant concentration in the N-band 80 ranges fromabout 5.0×10¹⁷ cm⁻³ to about 7.0×10¹⁸ cm⁻³. A thermal anneal may berequired to electrically activate the implanted impurities operating asthe p-type and n-type dopants.

An N-channel transistor 82 is built using the P-well 78, and a P-channeltransistor 84 is built using the N-well 78 to define a bulk CMOS device.The N-channel transistor 82 includes n-type diffusions in thesemiconductor material of substrate 40 representing a source region 86and a drain region 88 that flank opposite sides of a channel region inthe semiconductor material of substrate 40, a gate electrode 90overlying the channel region, and a gate dielectric 92 electricallyisolating the gate electrode 90 from the substrate 40. The P-channeltransistor 84 includes p-type diffusions in the semiconductor materialof substrate 40 representing a source region 94 and a drain region 96that flank opposite sides of a channel region in the semiconductormaterial of substrate 40, a gate electrode 98 overlying the channelregion, and a gate dielectric 100 electrically isolating the gateelectrode 98 from the substrate 40. Other structures, such as sidewallspacers (not shown), may be included in the construction of theN-channel transistor 82 and the P-channel transistor 84.

The conductor used to form the gate electrodes 90, 98 may be, forexample, polysilicon, silicide, metal, or any other appropriate materialdeposited by a CVD process, etc. The source and drain regions 86, 88 andthe source and drain regions 94, 96 may be formed in the semiconductormaterial of substrate 40 by ion implantation of suitable dopant specieshaving an appropriate conductivity type. The gate dielectrics 92, 100may comprise any suitable dielectric or insulating material like silicondioxide, silicon oxynitride, a high-k dielectric, or combinations ofthese dielectrics. The dielectric material constituting dielectrics 92,100 may be between about 1 nm and about 10 nm thick, and may be formedby thermal reaction of the semiconductor material of the substrate 40with a reactant, a CVD process, a physical vapor deposition (PVD)technique, or a combination thereof.

Processing continues to complete the semiconductor structure, includingforming electrical contacts to the gate electrodes 90, 98, source region86, drain region 88, source region 94, and drain region 96. The contactsmay be formed using any suitable technique, such as a damascene processin which an insulator is deposited and patterned to open vias, and thenthe vias are filled with a suitable conductive material, as understoodby a person having ordinary skill in the art. The N-channel andP-channel transistors 82, 84 are coupled using the contacts with otherdevices on substrate 40 and peripheral devices with a multilevelinterconnect structure consisting of conductive wiring and interleveldielectrics (not shown). A contact 102 is also formed in substrate 40that is electrically coupled with the N-well 76 for supplying thestandard power supply voltage (V_(dd)) to the N-well 76. Another contact104 is formed in substrate 40 for coupling the P-well 78 with thesubstrate ground potential.

In accordance with the principles of the invention, the lateralextensions 73, 75 of the bottom portion of the STI regions 74 increasethe base width or P-well path for the parasitic NPN structure 22(FIG. 1) and the base width or N-well path for the PNP structure 26(FIG. 1). As a consequence, holes traversing the N-well 76 to the P-well78, which constitutes the collector of the PNP structure 26, andelectrons traversing the P-well 78 to the N-well 76, which constitutesthe collector of the NPN structure 22, must flow around the lateralextensions 73, 75, which defines an inverted-T structure. A bump in theelectric potential forms at the lateral extensions 73, 75, due to theconcavity of the silicon surface about curved boundary 69 and boundingthe side edges 70, 72 of the lateral extensions 73, 75. This potentialbump impedes the flow of minority carriers and results in reduced betafor both parasitic NPN and PNP structures 22, 26. The damage regions 71in the semiconductor material of the substrate 40, if present, arebelieved to reduce the minority carrier lifetimes and to contribute tothe reduction of the bipolar gain of the parasitic NPN and PNPstructures 22, 26.

With reference to FIG. 7 in which like reference numerals refer to likefeatures in FIG. 2 and in accordance with an alternative embodiment ofthe present invention that does not rely on an ion implantation process,the anisotropic dry etching process transferring the trenches 48 fromthe patterned pad layer 44 into the underlying substrate 40 is halted atan intermediate base 106 that is shallower than base 54 (FIG. 2). Aconformal layer 108 of a dielectric material, such as 5 nm to 15 nm ofsilicon nitride deposited by a CVD process, is formed on the pad layer44 and the sidewalls 50, 52 and intermediate base 106 of trenches 48.

With reference to FIG. 8 in which like reference numerals refer to likefeatures in FIG. 7 and at a subsequent fabrication stage, the conformallayer 108 is anisotropically etched using, for example, an RIE or plasmaetching process that removes the material constituting the conformallayer primarily from horizontal surfaces selective to (i.e., with asignificantly greater etch rate than) the constituent semiconductormaterial of substrate 40. Un-removed portions of the conformal layer 108define spacers 110, 112 on the sidewalls 50, 52 of each shallow trench48.

Using the pad structure 42 and spacers 110, 112 as a mask, ananisotropic etching process is used to deepen the shallow trenches 48,which defines base 54. Respective surfaces 50 a, 52 a of thesemiconductor material of substrate 40 are exposed between base 54 andthe spacers 110, 112, as is the surface along base 54. The depthdifference between base 54 and intermediate base 106, which isdetermined based upon depths measured as a perpendicular distancerelative to surface 41, may be, for example, about 0.1 μm. The depthdifference also defines the vertical height of the surfaces 50 a, 52 aacross which the semiconductor material of substrate 40 borders theshallow trench 48 and, hence, is unmasked by spacers 110, 112. Theabsolute depths to which the shallow trenches 48 are etched may varyaccording to device design.

With reference to FIG. 9 in which like reference numerals refer to likefeatures in FIG. 8 and at a subsequent fabrication stage, an isotropicetching process is used to etch the semiconductor material of substrate40 bordering shallow trenches 48 exposed across the surfaces 50 a, 52 aand base 54. The isotropic etching process, which may be conducted in asingle etching step or multiple steps with different etch chemistries,selectively removes the semiconductor material of substrate 40vertically to slightly deepen base 54. The isotropic etching processalso removes the semiconductor material of substrate 40 laterally acrossthe surfaces 50 a, 52 a to define sidewalls 114, 116 that have a widerseparation than sidewalls 50, 52. During the isotropic etching process,the spacers 110, 112 mask and protect sidewalls 50, 52 above surfaces 50a, 52 a against removal. For example, the etching process may rely on anisotropic silicon etchant such as a wet or dry hydrofluoric acidetchant.

With reference to FIG. 10 in which like reference numerals refer to likefeatures in FIG. 9 and at a subsequent fabrication stage, spacers 110,112 are stripped from the sidewalls 50, 52 of each shallow trench 48using an appropriate etching process. A liner 118 is formed on thesidewalls 50, 52, sidewalls 114, 116, and base 54, as well as on the padlayer 44. The liner 118 may be, for example, silicon oxide grown byexposing the unmasked semiconductor material of substrate 40 to either adry oxygen ambient or steam in a heated environment. Another optionalliner (not shown) of, for example, silicon nitride may be applied as adiffusion barrier to prevent impurity migration from the trench fillmaterial into the semiconductor material of substrate 40 bordering theshallow trenches 48. The liner 118 also operates to repair any etchdamage incurred by the sidewalls 50, 52, sidewalls 114, 116, and base 54of each shallow trench 48.

With reference to FIG. 11 in which like reference numerals refer to likefeatures in FIG. 10 and at a subsequent fabrication stage, the shallowtrenches 48 are filled with an insulating or dielectric material, suchas HDP oxide or TEOS, deposited across the pad layer 44 and planarizedby, for example, a CMP process. The pad structure 42 and excess liner118 on the pad layer 44 are removed and planarized to define the STIregions 74 in the substrate 40 by a planarization process that makes thetop surface of the STI regions 74 substantially co-planar or flush withthe top surface 41 of substrate 40. Portions of the dielectric materialfill the concavities bounded by sidewalls 114, 116 to form the lateralextensions 73, 75 at the bottom of the STI regions 74.

With reference to FIG. 12 in which like reference numerals refer to likefeatures in FIG. 1 and at a subsequent fabrication stage, standard bulkCMOS processing follows as described above with regard to FIG. 6 to formthe N- and P-wells 76, 78, the N-band 80, the N-channel transistor 82,the P-channel transistor 84, and contacts 102, 104 in the substrate 40.A person having ordinary skill in the art will appreciate that thisembodiment of the present invention may be advantageously implemented ina dual-well CMOS structure that lacks the N-band 80.

With reference to FIG. 13 in which like reference numerals refer to likefeatures in FIG. 2 and in accordance with an alternative embodiment ofthe present invention that minimizes damage across a well junction 142(FIG. 15) between the subsequently-formed N- and P-wells 138, 140 (FIG.15) in a dual-well structure, a crystal damaging species is ionimplanted into the base 54 of the shallow trenches 48 before the STIregions 74 (FIG. 15) are defined by filling the shallow trenches 48 withdielectric material. Before implantation, the shallow trenches 48 arelined with a liner 121 consisting of one or more individual layers (notshown) of suitable materials, such as 1 nm to 3 nm of thermally grownsilicon oxide covered by 4 nm to 20 nm of silicon nitride depositedconformally by a CVD process.

Local crystalline damage regions 124, which include semiconductormaterial of substrate 40 that has been converted to anon-monocrystalline state and includes point and extended defects, areformed by introducing energetic ions, as indicated diagrammatically bysingled-headed arrows 122, by an ion implantation process into thesubstrate 40. The energetic ions 122, which are generated from a sourcegas, are directed to impinge the substrate 40 at normal or near-normalincidence. The ions 122 may originate from a source gas selected toprovide, when ionized and accelerated to impart kinetic energy, neutralimpurities in silicon like nitrogen (N), oxygen (O), carbon (C), gold(Au), platinum (Pt), germanium (Ge), and silicon (Si), and othersuitable elements capable of inducing lattice damage. The ions 122 maybe implanted with the substrate 40 at or near room or ambienttemperature, although the present invention is not so limited. The padstructure 42 masks underlying regions of the substrate 40 againstreceiving an ion dose during the ion implantation process such that onlydamage regions 124 of the substrate 40 are implanted with a significantdose of ions 122.

The trajectories of the ions 122 penetrate the substrate 40 across base54 of at least the shallow trench 48 that, after subsequent fabricationstages, intersects the well interface 142 (FIG. 15), as well asoptionally other trenches 48. The ions 122 lose energy via scatteringevents with atoms and electrons in the semiconductor materialconstituting substrate 40. Kinetic energy lost in nuclear collisionsdisplaces target atoms of the substrate 40 from their original latticesites and permanently damages the substrate 40. When each individual ion122 collides with a target atom of the substrate 40, a recoil cascade isinitiated that dissipates the transferred kinetic energy by collisionswith other target atoms. The recoil cascade generates vacancies andinterstitial atoms in the lattice structure of substrate 40 among theatoms in the lattice structure remaining on regular lattice sites.

The ions 122 eventually lose all of their initial kinetic energy andstop in the substrate 40 to produce one of the damage regions 124 ofnon-monocrystalline semiconductor material near the base 54 of eachshallow trench 48. The crystalline damage in the damage regions 124coincides approximately with the depth profile of the stopped ions 122.Similar to the stopped ions 122, each damage region 124 is characterizedby a depth profile distributed about a projected range, which ismeasured as a perpendicular distance of the damage peak from the topsurface 41, and having a range straggle. Essentially all of theimplanted ions 122 come to rest within a distance of three times therange straggle from the projected range, which implies that the damagehas a similar distribution. After the ion implantation is concluded,uncombined vacancies and interstitial atoms remain and are distributedacross the thickness of the damage regions 124, as well as extendeddefects. The depth profile of the implanted ions 122 and damage also hasa characteristic lateral straggle such that ions 122 and damage extendlaterally of the sidewalls 50, 52, as indicated generally by boundary126.

The ion dose is preferably selected such that the peak atomicconcentration of the implanted ions 122 in each damage region 124exceeds the solid solubility of the impurity in the constituent materialof the substrate 40. By exceeding the solid solubility, subsequentheated process steps do not anneal the crystalline defects in the damageregions 124. Advantageously, the peak atomic concentration for theimplanted ions 122 in each damage region 124 may be in the range of5×10¹⁹ cm⁻³ to 5×10²¹ cm⁻³ and, in certain embodiments, may be as low as5×10¹⁸ cm⁻³ to provide the requisite crystalline damage. For example, asuitable implanted ion dose may range from 1×10¹⁴ cm⁻² to 5×10¹⁶ cm⁻² ata kinetic energy between about 10 keV and about 50 keV, although theinvention is not so limited. The present invention contemplates otherimplant conditions, i.e., energy and dose, that are capable of formingthe damage regions 124 in substrate 40. The ions 122 are implantedacross the top surface 41 of the entire substrate 40, although certainregions of substrate 40 may be optionally protected by a block mask (notshown) during implantation.

With reference to FIG. 14 in which like reference numerals refer to likefeatures in FIG. 13 and at a subsequent fabrication stage, a pattern ofdeep trenches 128 is formed in the substrate 40 by a conventionallithography and subtractive etching process. To that end, a photoresist130 is applied on pad layer 44 and exposed to a pattern of radiationthat, after developing, creates a deep trench pattern. An anisotropicdry etching process, such as reactive-ion etching (RIE) or plasmaetching, may then be used to transfer each deep trench 128 from the deeptrench pattern in the patterned photoresist 130 into the substrate 40.The deep trench pattern in the photoresist 130 is tailored such thateach deep trench 128 is registered with a corresponding one of theshallow trenches 48 overlying the future location of the well junction142 (FIG. 15).

The damage region 124 (FIG. 13) coinciding with the shallow trench 48associated with deep trench 128 is partially removed by the anisotropicdry etching process forming the deep trench 128. As a result, sidewalls127, 129 of the deep trench 128 are each flanked by a corresponding oneof a pair of damage regions 132, 134. The deep trench 128 separatesdamage regions 132, 134 so that the crystalline damage in thesemiconductor material of substrate 40 (i.e., the non-monocrystallinesemiconductor material) is discontinuous and interrupted across the welljunction 142 (FIG. 15). Specifically, a bottom or base 131 of the deeptrench 128 is at a greater depth than the damage regions 132, 134. Thesidewalls 127, 129 are narrower than the sidewalls 50, 52 of thecorresponding shallow trench 48 and the base 131 is at a greater depth,measured perpendicular to surface 41, than base 54 of the correspondingshallow trench 48.

With reference to FIG. 15 in which like reference numerals refer to likefeatures in FIG. 14 and at a subsequent fabrication stage, residualphotoresist 130 (FIG. 14) is stripped by, for example, plasma ashing ora chemical stripper after the deep trenches 128 are etched. The shallowtrenches 48 are filled with amounts of an insulating or dielectricmaterial, such as HDP oxide or TEOS, deposited across the pad layer 44and planarized by, for example, a CMP process. The pad structure 42 isremoved and planarized to define the STI regions 74 in the substrate 40by a planarization process that makes the top surface of the STI regions74 substantially co-planar or flush with the top surface 41 of substrate40. Portions of the dielectric material also fill the deep trench 128 todefine a pigtail or extension 136 that separates the damage regions 132,134. Standard bulk CMOS processing follows as described above withregard to FIG. 6 to form N- and P-wells 138, 140, similar to N- andP-wells 76, 78 (FIG. 6), the N-channel transistor 82, the P-channeltransistor 84, and contacts 102, 104 in the substrate 40. A personhaving ordinary skill in the art will appreciate that this embodiment ofthe present invention may be advantageously implemented in a triple-wellCMOS structure that includes an N-band (not shown) similar to N-band 80(FIG. 6).

The selectively introduced lattice damage reduces the current gains ofthe parasitic NPN and PNP structures 22, 26 (FIG. 1) without degradingwell leakage of the N- and P-wells 138, 140. Ordinarily, crystal damageacross well junction 142 between the N- and P-wells 138, 140 causes adepletion region in a dual-well bulk CMOS technology, which increasesthe well leakage currents. In accordance with the present invention, therelatively narrow deep trench 128 is aligned relative to thecorresponding shallow trench 48 to intersect the well junction 142between the N-well 138 and P-well 140, thus removing the damagedsemiconductor material across the well junction 142. Thus, the damage indamage regions 132, 134 exists only within the portion of the N- andP-wells 138, 140 that constitutes the base of the parasitic NPN and PNPstructures 22, 26 (FIG. 1). The crystal damage in the base regionsshortens the minority carrier lifetime of the carriers emitted by theemitters and, thereby, reduces the bipolar gain to the point wherelatch-up is not sustained. Because the damage is located away from thewell junction 142, well leakage is not degraded.

With reference to FIG. 16 in which like reference numerals refer to likefeatures in FIG. 14 and in accordance with an alternative embodiment ofthe present invention, ions 152, similar or identical to ions 122 (FIG.13), may be directed into the sidewalls 127, 129 of the deep trench 128.An implantation mask 154 of, for example, HDP oxide is applied to selfalign the impinging ions 152 to the sidewalls 127, 129 and to preventimpinging ions 152 from entering the semiconductor material of thesubstrate 40 near the base 131 of the deep trench 128. A portion of theimplantation mask 154 masks the base 131 of the deep trench 128, whichprevents damage to the well junction 142. The implanted ions 152 formdamage regions 156, 158 in the semiconductor material of substrate 40that are similar or identical to damage regions 132, 134 (FIG. 14).These damage regions 156, 158 of non-monocrystalline semiconductormaterial may be used in conjunction with damage regions 132, 134 forsuppressing latch-up. Processing continues as shown in FIG. 15 tocomplete semiconductor structure.

With reference to FIG. 17 in which like reference numerals refer to likefeatures in FIG. 13 and in accordance with an alternative embodiment ofthe present invention, a high defect region may be produced near thebase 54 of the shallow trenches 48 without ion implantation. To thatend, protective spacers 144, 146 of an insulating material, such assilicon oxide or silicon nitride, are formed on the sidewalls 50, 52 ofat least the shallow trench 48 that, after subsequent fabricationstages, intersects the well junction 142.

A layer 148 of a semiconductor material, such as SiGe, having a latticemismatch with the semiconductor material of the substrate 40 is thendeposited or grown at the bottom of the shallow trench 48. Theprotective spacers 144, 146 guard the sidewalls 50, 52 against theformation of an extraneous layer (not shown) of the materialconstituting layer 148 on sidewalls 50, 52. The lattice mismatch orcrystal lattice constant difference between the materials in layer 148and substrate 40 results in a region 150 of high carrier recombinationvelocity in the substrate 40 beneath the shallow trench 48. Region 150is characterized by a high recombination velocity and getters orattracts carriers in transit to the collectors of the parasitic NPN andPNP structures 22, 26 (FIG. 1). Processing continues as shown in FIG. 14to complete the semiconductor structure (FIG. 15).

With reference to FIG. 18 in which like reference numerals refer to likefeatures in FIG. 3 and in accordance with an alternative embodiment ofthe present invention, the oxygen implanted regions 58 (FIG. 3) may beremoved with an appropriate isotropic etching process to leave opencavities or voids 59. The open voids 59, which communicate with acorresponding one of the shallow trenches 48, are each filled withdielectric material when the shallow trenches 48 are filled. Thedielectric-filled open voids 59 define the lateral extensions 73, 75(FIG. 4). Processing continues as shown in FIG. 4 to complete thesemiconductor structure.

References herein to terms such as “vertical”, “horizontal”, etc. aremade by way of example, and not by way of limitation, to establish aframe of reference. The term “horizontal” as used herein is defined as aplane parallel to the top surface 41 of substrate 40, regardless of itsactual spatial orientation. The term “vertical” refers to a directionperpendicular to the horizontal, as just defined. Terms, such as “on”,“above”, “below”, “side” (as in “sidewall”), “higher”, “lower”, “over”,“beneath” and “under”, are defined with respect to the horizontal plane.It is understood that various other frames of reference may be employedfor describing the present invention without departing from the spiritand scope of the present invention.

The fabrication of the semiconductor structure herein has been describedby a specific order of fabrication stages and steps. However, it isunderstood that the order may differ from that described. For example,the order of two or more fabrication steps may be switched relative tothe order shown. Moreover, two or more fabrication steps may beconducted either concurrently or with partial concurrence. In addition,various fabrication steps may be omitted and other fabrication steps maybe added. It is understood that all such variations are within the scopeof the present invention. It is also understood that features of thepresent invention are not necessarily shown to scale in the drawings.

While the present invention has been illustrated by a description ofvarious embodiments and while these embodiments have been described inconsiderable detail, it is not the intention of the applicants torestrict or in any way limit the scope of the appended claims to suchdetail. Additional advantages and modifications will readily appear tothose skilled in the art. Thus, the invention in its broader aspects istherefore not limited to the specific details, representative apparatusand method, and illustrative example shown and described. Accordingly,departures may be made from such details without departing from thespirit or scope of applicants' general inventive concept.

1. A semiconductor structure comprising: a substrate of a semiconductormaterial having a top surface; a first doped well formed in thesemiconductor material of the substrate, the first doped well having afirst conductivity type; a first field effect transistor with source anddrain regions in the first doped well; a second doped well formed in thesemiconductor material of the substrate and disposed adjacent to thefirst doped well, the second doped well having a second conductivitytype; a second field effect transistor with source and drain regions inthe second doped well; and a dielectric-filled trench defined in thesubstrate between the first and second doped wells, the trench includinga base, first sidewalls intersecting the top surface, and secondsidewalls disposed between the base and the first sidewalls, the secondsidewalls having a wider separation than the first sidewalls, and thewider separation of the second sidewalls operating to reduce latch-up ofthe first and second field effect transistors.
 2. (canceled)
 3. Asemiconductor structure comprising: a substrate of a semiconductormaterial having a top surface; a first doped well formed in thesemiconductor material of the substrate; a second doped well formed inthe semiconductor material of the substrate and disposed adjacent to thefirst doped well; and a dielectric-filled trench defined in thesubstrate between the first and second doped wells, the trench includinga base, first sidewalls intersecting the top surface, and secondsidewalls disposed between the base and the first sidewalls, the secondsidewalls having a wider separation than the first sidewalls, whereinthe semiconductor material of the substrate is monocrystalline, and thesemiconductor material of the substrate bordering the second sidewallsand the base includes a damage region of non-monocrystallinesemiconductor material.
 4. A semiconductor structure comprising: asubstrate of a semiconductor material having a top surface; a firstdoped well formed in the semiconductor material of the substrate; asecond doped well formed in the semiconductor material of the substrateand disposed adjacent to the first doped well, the first doped well andthe second doped well having a first conductivity type; a third dopedwell formed in the semiconductor material of the substrate, the thirddoped well arranged between the first doped well and the top surface,the third doped well having a second conductivity type that differs fromthe first conductivity type; and a dielectric-filled trench defined inthe substrate between the first and second doped wells, the trenchincluding a base, first sidewalls intersecting the top surface, andsecond sidewalls disposed between the base and the first sidewalls, thesecond sidewalls having a wider separation than the first sidewalls. 5.The semiconductor structure of claim 4 further comprising: a first fieldeffect transistor with source and drain regions in the first doped well;and a second field effect transistor with source and drain regions inthe third doped well, the wider separation of the second sidewallsoperating to reduce latch-up of the first and second field effecttransistors.
 6. The semiconductor structure of claim 5 wherein the firstand second doped wells have an n-type conductivity, the third doped wellhas a p-type conductivity, the source and drain regions of the firstfield effect transistor comprises p-type diffusions, and the source anddrain regions of the second field effect transistor comprise n-typediffusions.
 7. A semiconductor structure comprising: a substratecomprising a monocrystalline semiconductor material having a topsurface; a first doped well formed in the semiconductor material of thesubstrate; a second doped well formed in the semiconductor material ofthe substrate and disposed adjacent to the first doped well along a welljunction; and a dielectric-filled trench defined in the substratebetween the first and second doped wells, the trench including a base,first sidewalls intersecting the top surface, and second sidewallsextending from the base to the first sidewalls, the second sidewallshaving a narrower separation than the first sidewalls, wherein thesemiconductor material of the substrate bordering the second sidewallsincludes a damage region comprising non-monocrystalline semiconductormaterial and the base of the trench is at a greater depth than thedamage region for interrupting the continuity of the non-monocrystallinesemiconductor material across the well junction.
 8. A semiconductorstructure comprising: a substrate of a first material characterized bysemiconducting properties and having a top surface; a first doped wellformed in the substrate; a second doped well formed in the firstmaterial of the substrate and disposed adjacent to the first doped well;a trench defined in the substrate between the first and second dopedwells, the trench including a base and sidewalls intersecting the topsurface; a dielectric material filler in the trench; and a layer of asecond material disposed between the first material at the base of thetrench and the dielectric material filler, the first and secondmaterials having a crystal lattice constant difference sufficient toincrease carrier recombination velocity.
 9. The semiconductor structureof claim 8 wherein the first material comprises silicon and the secondmaterial comprises an alloy of silicon and germanium. 10-26. (canceled)